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'As cast' skin of silicon bronze is dull

The 'as cast' skin of silicon bronze is rather dull if the casting is done properly, metal poured warm, mold free enough from carbon. The skin is grey/black is little bit powdery with some remnants of the casting mold attached. The black layer is black copper oxide which is loosely attached to the surface. After high pressure water cleaning this layer is largely dissapeared and the reddish conversion layer becomes visible mottled with remnants of the black layer. The red colour comes from red copper oxide (cuprite), the oxidation of copper goes in two steps, from red to black. .

Spots with shrinkage have a xx colour.

If you are an average artist you will probably prefer another alloy, cast with 555, very hot in a mold which did not have a proper burn out. You will love the results. A disadvantage might be that several castings are needed to get the right mix of flaws.

90/10 is loved because the reddish tones. The most used method is to pour water over the mold a short time after the pouring when the metal is still hot, 500 degrees C. The hot demolding leads on fragile statues often to cracks which cannot be repaired invisible.